LOW TC CONDUCTOR DEVELOPMENT FOR ELECTRIC UTILITY DEVICES

Super-GM-Supported Conductor Development

The Super-GM project (see also Chapter 2) supports conductor development projects for eventual use in stators of full-sized SC generators, practical reactors (e.g., 66 kV/100 MVA), fault current limiters, etc. Various manufacturers are developing both NbTi and Nb₃Sn wires, as listed above. Figure 6.1 shows a typical cross-section of a conductor and a wire and a schematic for construction of a low ac loss NbTi cable. The cross-section of the basic wire is heavily segmented with a high-resistance NiCu alloy, which occupies approximately 80% of the wire. Most of the remainder is fine filamentary (filament diameter < 0.2 mm ) NbTi and a small amount, if any, of Cu (0-3%). All of this is done to reduce the ac losses at power frequencies. The I_c performance of a conductor for armature windings is approximately 3,000 A_rms at 1.5 T and 4.2 K. The goal of the project is to produce conductors that can carry an I_c of several kA_rms at 1.5 T. This wire development follows earlier projects, which were supported by Japan's Ministry of Education (MOE), on construction of transformers at a number of universities. The largest of these transformers was built at Kyushu University with a design power rating of 1,000 kVA, but the conductor for the transformer was able to carry only 60% of the rated current (4.5 kA_rms), although the small-scale test of the conductor met all the design specifications. The source of this degradation of I_c for large conductors is still not known. Thus, the main goal of the Super-GM project for NbTi conductors is to increase the total current capacity under realistic conditions for power device applications.

Fig. 6.1. Cross-sectional pictures of typical NbTi wire and cable used for electrical utility devices such as transformers: a schematic diagram for construction of a wire for use at power frequencies (first), a cross-section of a finished wire (second), and a third stranded cable (third).

The Super-GM project also supports extensive development of ac conductors using Nb₃Sn in order to take advantage of its higher T_c and associated better thermal stability. All of the major manufacturers of superconductors are participating in the project (Table 6.2). In contrast to NbTi wire, Nb₃Sn wire can be fabricated in a number of ways. Although the so-called "bronze" and "internal tin" processes are the most commonly used methods for large-scale production of wire, apparently wires have also satisfactorily been produced by other methods that meet two of the project's preliminary goals: ac loss <= 25 kW/m³, and I_c > 1 kA at 0.5 T and 4.2 K. However, final target levels for I_c and ac losses for Nb₃Sn have not been reported.

CRIEPI-Supported Conductor Development

Outside of the Super-GM project, the Central Research Institute for the Electric Power Industry (CRIEPI), in collaboration with Nihon University and Showa Electric Wire and Cable Company, is developing Nb₃Sn wires for ac applications by using a novel approach to the internal tin process. As will be discussed below in connection with conductors for ITER, the effective filament size in the internal tin processed wire is generally much greater than a few micrometers. However, in order to reduce ac losses at power frequencies, the size of the filaments must be well below one micrometer, and this is not usually achieved in the standard high field wires. Part of the reason for this is that the filaments tend to coalesce during high temperature (~ 700°C) heat treatment for forming Nb₃Sn. What the CRIEPI group has found is that for ac applications the heat treatment temperature can be quite low, e.g., 450-500°C, to react all of the Nb filaments to Nb₃Sn, if the filaments are drawn down to submicron (<= 0.3 µm) dimensions. This low heat treatment temperature can keep the coalescence of the filaments to a minimum, even though the value of T_c (~ 14 K) of the Nb₃Sn produced is substantially lower than what is achieved in a standard heat treatment. Small coils have been made, and they were being tested in mid-1996 for their ac excitation performance.

Electric Utility-Supported Conductor Development

Another source of support for development of low T_c superconductors is the electric power utilities. For example, Tokyo Electric Power Company (TEPCO) has been supporting development of a fault-current limiter with NbTi wire. In this application, ac losses at power frequencies are a primary factor in the design, and the conductor for the fault-current limiter requires a characteristic similar to those for power transformers. It was not clear to the WTEC panel who had developed the wire for the limiter, but it is likely that the TEPCO project supported the wire development component.